The invention relates to a process for electrochemically gasifying coal and, more particularly, to a process by which either hard or soft coal can be reduced to combustible gases that are relatively free of other gaseous contaminants, such as sulfur content gases that are typically produced in known prior art coal gasification processes.
The advantages of producing synthetic gases from solid fuels, such as coal or peat, have long been well known. Relatively greater efficiencies realizable in the utilization of gaseous fuels versus solid fuels, coupled with the availability of large reserves of coal, peat and tar sands or shale, have provided strong impetus in recent years for the development of more commercially feasible processes for reducing solid fuels to gaseous fuels.
In the context of prior art coal gasification processes, the term "gasification" is ordinarily used to refer to processes that use high gasification temperatures, i.e. temperatures in the order of 1000.degree. C. or more. When hard coal is gasified at such temperatures nearly complete conversion of the organic mass of the coal into combustible gases is achieved by supplying oxygen, hydrogen, water or carbon dioxide to the coal while it is maintained at the elevated temperature and under a suitable pressure. In addition to the carbon monoxide, carbon dioxide and hydrogen gases that are produced in such processes, it is also normal to find nitrogen, hydrogen sulfide and COS gases present in the output of such coal gasification processes. The presence of such undesirable contaminant gases usually requires the use of subsequent cleaning and conversion steps to either separate or filter out the undesired gases, or to crack the hydrocarbons in a suitable methanation reaction, thereby to form methane and water. These limitations have, heretofore, prevented the development of any suitably high volume, commercially feasible processes for producing inexpensive and relatively pure fuel gases from coal.
Up until about the mid 1970's, there existed three well known processes for achieving coal gasification. Of those processes, the one most widely now used by industry is the so called Lurgi process. In the Lurgi process a stationery fuel or so-called fixed bed fuel is used to achieve coal gasification at a predetermined pressure. It is now generally known that the heating value of gases thus produced can be improved by increasing the pressure under which the coal is gasified.
A somewhat older process, known as the Winkler process, was introduced about the year 1926. In the Winkler process either hard or soft coal is gasified in a moving or fluidized bed that is maintained at ambient pressure. This process produces high gas yields relative to those from the Lurgi process, but also requires high gasification temperatures and subsequent cleaning, filtering or cracking of the resultant gases, in order to obtain essentially pure hydrogen and carbon dioxide product gases.
A refinement of the Winkler process was developed in the early 1950's and is called the Koppers-Totzek process. In that process coal must be ground into very fine particulate before it is gasified at ambient pressure. This process is more efficient than the two earlier-mentioned processes, but it also requires gasification temperatures of about 2000.degree. C., and it produces gases that must be filtered, cleaned or cracked in order to secure relatively pure hydrogen and carbon dioxide gases.
In addition to these three basic coal gasification processes, each of which is in fairly widespread usage, a number of additional gasification processes were developed, at least to the prototype or experimental stage, during the 1970's and early 1980's. Although it is difficult to generally characterize these more recent gasification processes, many of them are operated by reacting pure hydrogen in a hydropyrolysis reactor in order to produce methane. The earlier of these prototypical processes normally use a selected synthetic gas (syngas) in such a reaction step, to produce the desired methane. For the methanation step in such a reactor, the later of these recent processes also require secondary process steps, such as the generation of hydrogen for the methanation reaction which is usually done by char gasification and an associated shift conversion and removal of carbon dioxide. Subsequently, it is necessary to remove the excess hydrogen that is required in the methanation step. It is known that such separation can be achieved cryogenically so that the excess hydrogen can be recycled to the gasifier.
Besides such potentially high volume coal gasification processes, some small-scale experiments have been conducted to determine the feasibility of electrochemically gasifying coal to produce directly relatively pure hydrogen, carbon dioxide and water. An advantage of this technique is that it eliminates the difficulties and expenses associated with cleaning and filtering or cracking the contaminant gases normally found in the commercial coal gasification processes. Some of the test results achieved with such electrochemical processes are discussed by Messrs. R. W. Coghlin and M. Farooque in a paper entitled "Electrochemical Gasification of Coal . . . ", which was published during 1980 by the American Chemical Society, in Industrial Engineering Chemical Process Design Development, at pages 211-219, Vol. 19, No. 2 (1980). The tests reported in that paper indicate that a recently developed electrochemical process operates to convert coal and water into two separate gaseous products comprising essentially gaseous oxides of carbon and pure hydrogen. The chemical reaction takes place at mild temperature, such as normal room temperatures, and the gaseous products are essentially free of impurities such as ash, tar and sulfur compounds of the type normally associated with other prior art coal gasification processes. In the type of electrochemical gasification cell described in that paper, the desired gasification is achieved by applying a relatively low electric potential, around 0.21 volts, to a coal water slurry contained in a beaker in which two platinum mesh electrodes are suspended. The tests described also established that by increasing the coal-to-electrolyte loading within the cell it was possible to cause about a ten-fold increase in oxidation current. The influences of temperature and electrode potential, as well as those of other process parameters, were briefly explored, as functions of their effects on activation temperature.
In another paper entitled, "On the Electrolysis of Coal Slurries", authored by Messrs. G. Okada, V. Guruswany and J. Brockris of the Department of Chemistry, Texas A&M University, and published in the Journal of the Electrochemical Society, Vol. 128, No. 10, October 1981, the results of experiments which establish the feasibility of electrolyzing coal slurries to produce essentially pure CO.sub.2 and H.sub.2 are described. In the tests reported in that paper, a simple electrolysis cell utilizing two electrodes immersed in a coal slurry and electrolyte mixture was used to achieve a successful electrolysis of coal.
Although the operability of such electrolysis reactions are of interest in studying alternative coal gasification processes, it is apparent that in order to adapt their teachings for possible use in suitably high volume commercial gasification process apparatus some additional means would be necessary for suitably protecting the electrodes from becoming quickly poisoned or blocked by evolved gases. Also, it would be necessary to develop means for economically supplying electricity to electrodes that could present high surface area exposure to the coal slurry and electrolytes used in practicing such processes. The foregoing papers only broadly describe the types of apparatus that might be usable to achieve such high volume, continuous electrochemical coal gasification process applications.